As an image forming material (hereinafter referred to also as a light sensitive material) comprising a positive working light sensitive layer to be solubilized by actinic light irradiation, an image forming material comprising a light sensitive layer containing an acid generating compound and an acid decomposable compound is known. That is, a light sensitive composition containing an acid generating compound and a water insoluble compound having a specific group capable of decomposed by an acid is disclosed in U.S. Pat. No. 3,779,779, a light sensitive composition containing an acid generating compound and a compound having an acetal or a ketal in the main chain is disclosed in Japanese Patent O.P.I. Publication No. 53-133429/1978, and a light sensitive composition containing an acid generating compound and a compound having a silylether group is disclosed in Japanese Patent O.P.I. Publication No. 65-37549/1985. These compositions have sensitivity in the ultraviolet range, which are capable of being alkali solubilized by imagewise ultraviolet ray exposure to provide non-image portions at exposed portions and to provide image portions at non-exposed portions. The imagewise ultraviolet ray exposure is generally carried out through a mask film by employing ultraviolet rays emitted from a light source such as a halogen lamp or a high pressure mercury lamp, or can be carried out by employing a short wavelength laser such as an argon laser or a helium-cadmium laser. However, these light sources are expensive, and troublesome in using due to its large size. Further, the above described light sensitive materials could not necessarily provide a satisfactory result in view of sensitivity.
In order to increase sensitivity of a light sensitive layer, to improve a chemical resistance of a light sensitive layer to various cleaners for cleaning a printing surface, or to improve an aptitude of a light sensitive layer to a ball-point pen ink, the present inventors have proposed a positive working light sensitive material controlling a dipole moment by adding a specific polymer to a light sensitive layer (Japanese Patent Application No. 9-5315) or a positive working light sensitive material comprising an o-quinonediazide compound and a polymer containing an aromatic monomer unit (Japanese Patent Application No. 6-329987). However, the imagewise exposure employing ultraviolet rays still has the problems as described above. A simple and less expensive exposure method has been eagerly sought.
A technique for forming an image by irradiation of infrared rays such as a cheap and compact semiconductor laser is proposed. In U.S. Pat. No. 5,340,699 is disclosed an image forming material comprising a light sensitive layer containing an acid generating compound, a resol resin, a melamine resin, a novolak resin and an infrared absorber, wherein a negative image is formed by a method comprising imagewise exposing the material to infrared rays, heating the exposed material before development, and then developing the heated material, or a positive image is formed by a method comprising imagewise exposing the material to infrared rays, and then developing the exposed material without heating. However, the method forming a negative image requires the heat treatment, resulting in much electrical power consumption or more load to the processor. The method forming a positive image has a problem in that the light sensitive layer contains much of a residual solvent and the light sensitive layer at image portions is likely to be partially or entirely dissolved in a developer. For example, when a presensitized planographic printing plate (one embodiment of the image forming material of the invention) comprising the above described light sensitive layer is imagewise exposed to infrared rays and developed with a developer, the layer at image portions may be damaged by the developer, although the damage depends upon the concentration of the developer. That is, there may occur a phenomenon called "layer damage", which is caused due to low resistance of the layer to developer. Further, the developed plate (printing plate), which is obtained from the presensitized planographic printing plate, has a problem in that the light sensitive layer at image portions may be damaged by chemicals used during printing due to low resistance to chemicals used during printing. Furthermore, sensitivity fluctuation after long-term storage is large, and the conventional image forming material is not satisfactory in view of storage stability.